The wet milling of corn makes available four principal products: starch, gluten feed, gluten meal, and oil. In general, the purposes of the wet milling process are to separate starch and oil in pure form for food use, to isolate a large percentage of the protein in a concentrated form (gluten meal) for food or feed use, and to combine residual fibrous and soluble fractions to produce a feed ingredient (gluten feed) of about 21% protein content. (A complete description of the milling process may be found in the Kirk--Othmer Encyclopedia of Chemical Technology, 3rd Edition, Volume 21, p. 492-507, 1983, herein incorporated by reference.)
The several steps of the wet milling process include first passing the corn kernels through mechanical cleaners designed to separate unwanted substances, e.g., cob, sticks, husks, metal and stone, followed by soaking (steeping) the corn for 24 to 48 hours in circulating warm water containing a small amount of sulfur dioxide to inhibit putrefactive fermentation and facilitate softening. The kernels swell appreciably in the steeping; when fully softened, they contain 40% to 50% moisture, their hulls are somewhat loosened, and they are ready for much easier separation of their component parts than would be possible otherwise.
The softened kernels are then degerminated by a milling action in which the rotary action of one plate pressing the kernel against a similar, stationary plate tears the soft kernel apart to free the rubbery germ without crushing it, and with concomitant liberation of a portion of the starch and protein from the endosperm. The germs, being lighter than the other parts of the kernels, float to the surface of the aqueous mixture where they may be skimmed off or, in the case of modern milling plants, separated by hydroclones.
This leaves an aqueous slurry of starch, gluten, fiber and chunks of the softened starch endosperm. Excess water and much of the free starch and protein that were liberated coincident with degermination are separated by screening; the remaining mixture of endosperm chunks and fibrous materials is ground or impacted to a fine mash and washed on a series of screens to retain the fibrous material and hard proteinaceous endosperm particles that resist comminution; the starch and gluten pass through the screen of metal or fabric composition which retains the fibrous and proteinaceous material of largest particle size.
This now leaves only the starch and gluten as an aqueous slurry. This slurry is pumped from the shakers to high speed centrifugal machines. Because of the difference in specific gravity, the relatively heavier starch is easily separated from the lighter gluten by centrifugal force, yielding a gluten fraction and a starch fraction.
The fiber, germ and gluten fractions are obtained from the main slurry stream of the wet milling process as separated aqueous slurries which are to be dewatered. This water is subsequently removed via mechanical and thermal processes to achieve the desired moisture content of the final products. The dried fiber and gluten are sold directly as products. The dried germ is further processed to recover and refine the corn oil retained within.
The starch recovered in the wet milling process is further processed into other products; many starches are chemically modified to achieve specific functionality requirements. The bulk of the starch, however, is hydrolyzed into simple sugar (dextrose), then refined into dextrose-bearing corn syrups or further converted and refined to produce fructose-bearing corn syrups, both in liquid and crystalline form.
Maximizing starch recovery (or yield) is a primary objective of wet milling plants. The wet milling process does not allow for complete recovery of the starch contained in the kernels because of inefficiencies of the mechanical separation operation and the steeping process. The latter process (steeping) is primarily diffusion limited, while the former process has reached hydraulic and mechanical limitations through a continual improvement effort by the producers.